Merge bitcoin/bitcoin#24507: fix CI: bitcoin-chainstate: Lock `cs_main` to `UnloadBlockIndex`

7a68fe4831 bitcoin-chainstate: Lock cs_main to UnloadBlockIndex (Carl Dong)

Pull request description:

  This was introduced because of a silent merge conflict.

ACKs for top commit:
  promag:
    ACK 7a68fe4831
  jonatack:
    ACK  7a68fe4831

Tree-SHA512: 4c135efd68604452485a129e731675ff5917c157a70c77dd702211d9902c21b3b29380a881723f43ecba4762bc864b036881bb502b3b792e581565dcaa7a7ed4

src/bitcoin-chainstate.cpp

@@ -256,7 +256,7 @@ epilogue:
     }
     GetMainSignals().UnregisterBackgroundSignalScheduler();
 
-    UnloadBlockIndex(nullptr, chainman);
+    WITH_LOCK(::cs_main, UnloadBlockIndex(nullptr, chainman));
 
     init::UnsetGlobals();
 }